I use Nostur and if I tap on a profile pic it gives a little popup that includes “last seen” which essentially calculates the last time you signed an event to a relay i’m connected to. I’m not sure if there’s one that shows it in a more consolidated view for all your follows though. That would be helpful. 

https://image.nostr.build/aad32106c2c99b89ba43f1048cbe839185091c0d5c722231168a04f474825807.jpg